Small water craft, meaning inflatable boats and rigid hulled boats of up to about eight meters, are usually powered by one (or more) outboard motors attached to a rear transom. The drive shaft of the motor is substantially vertical. At the lower end of the drive shaft there is a gear box through which drive is transmitted to a substantially horizontal propeller shaft. The main advantage of an outboard motor is that the power losses in the drive train between the crank shaft and the propeller shaft are small. The main disadvantages of an outboard motor are that it occupies space at the stern end of the deck thereby reducing available deck space, it is noisy and is unsightly.
Inboard motors are rarely found on the smallest boats where outboards are used almost exclusively. Larger boats use almost exclusively inboard engines. However, there is a size range (say from four meters to eight meters) where some boats have outboard motors and others have inboard motors. An inboard motor is entirely concealed within the structure of the hull and the boat is thus more aesthetically pleasing. The main disadvantage of the inboard motor is that the crank shaft of the motor rotates about a horizontal axis and the propeller shaft also rotates about a horizontal axis but at a lower level. Hence two sets of gearing, usually bevel gearing, and an intermediate shaft, are required to form a power train from the crank shaft to the propeller shaft. Power losses are hence substantial. For this reason inboard and outboard motors of the same rated power will provide substantially different powers at the propeller. As much as thirty percent more power can be lost in the drive train of an inboard motor than in the drive train of an outboard motor.
An inboard motor is approximately twice the weight of an outboard motor for the same power.
Motor mountings have been proposed in which the motor's block is within the hull of the boat, the drive shaft passing through a transom and there being a gear box and propeller externally of the hull. An example of a motor mounting of this type is found in U.S. Pat. No. 3,382,839.
The main object of the present invention is to provide a motor mounting which provides the boat with advantageous handling characteristics.